1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor layer having a crystalline structure formed on a substrate having an insulation surface, and in particular a method for manufacturing a semiconductor device in which the corresponding semiconductor layer is used in an active layer. Especially, the invention is oriented at a method for producing a thin film transistor in which an active layer is formed from a crystalline semiconductor layer. Also, in the specification, a “semiconductor device” includes all types of devices which can function by utilizing semiconductor characteristics, wherein the category thereof includes electro-optical devices represented by an active matrix type liquid crystal display device formed by using a thin film transistor, and an electronic apparatus in which such an electro-optical device is incorporated.
2. Description of Related Art
A thin film transistor (hereinafter called “TFT”) has been developed, in which an amorphous semiconductor layer is formed on an insulation substrate having a light transmission property such as glass, and a crystalline semiconductor layer which is crystallized by a laser annealing method and a thermal annealing method, etc. In many cases, glass substrates made of barium borosilicate glass and aluminum borosilicate glass, etc., are used. Since the market price of such glass substrates are inexpensive, though they are inferior to quartz substrates in terms of heat-resistance, such glass substrates have an advantage in that large-area substrates can be easily produced.
The laser annealing method has been known as a crystallizing technique by which amorphous semiconductor layers can be crystallized by providing a high energy onto only the amorphous semiconductor layers without raising the temperature of the glass substrates. In particular, it is considered that an excimer laser which is capable of providing a large output of short wavelengths is optimal. The laser annealing using an excimer laser is carried out by processing the laser beam so as to, by an optical system, make the beam spot-like or linear on a surface to be irradiated, and by scanning with the processed laser beam on the irradiated surface (relatively moving the irradiation position of the laser beam with respect to the irradiated surface). For example, an excimer laser annealing method in which a linear laser beam is used enables laser annealing on the entire irradiated surface by scanning in a direction orthogonal to the lengthwise direction thereof. Since the productivity thereof is excellent, the laser annealing method becomes the main stream as the production technology of a liquid crystal display device using TFT. The technology enabled a monolithic type liquid crystal display device in which a pixel TFT forming a pixel portion and a TFT of a drive circuit secured around the pixel portion are formed on a single glass substrate.
However, the crystalline semiconductor layer produced by the laser annealing method is formed by gathering a plurality of crystalline grains, and the position and size of the crystalline grains are random. TFTs produced on the glass substrate are formed so that the crystalline semiconductor layer is separated into island-like patterns for the separation of elements. In this case, the TFT could not be formed while specifying the positions and sizes of the crystalline grains. On the boundary of crystalline grains (crystalline grain phase), a lowering of the current carrying characteristics of a carrier was caused by the influences of a potential level at the re-coupling center, capturing center and crystalline grain boundary resulting from the amorphous structure and crystallizing defect. But, it is almost impossible that a channel forming region in which the crystalline characteristics seriously influence the characteristics of the TFTs is formed with mono-crystalline grains by excluding the influences of the crystalline grain boundary. Therefore, the TFTs, in which a crystalline silicon film is used as an active layer, having characteristics equivalent to those of a MOS transistor produced on a mono-crystalline silicon substrate could not be obtained until today.
In order to solve these problems, an attempt to increase the crystalline grain size has been made. For example, in “High-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Method”, prepared by K. Shimizu, O. Sugiura and M. Matsumura, IEEE Transactions on Electron Devices, Vol. 40., No. 1, pp112-117, 1993, a report is given of a dual beam laser annealing method in which a film of three layered (Si/SiO2/Si) structure is formed, and an excimer laser beam is irradiated from both the film side and the substrate side. According to the method, by irradiating a laser beam at a specified energy intensity, it is possible to increase the crystalline grain size.
In a monolithic type liquid crystal display device, a pixel portion for performing image display and a drive circuit are formed on the same substrate. The pixel portion is provided with a pixel TFT and a holding capacitor while the drive circuit is provided with a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, etc., which are formed on the basis of a CMOS circuit. However, the operating conditions of the pixel TFTs are not the same as those of the drive circuit TFTs, whereby the characteristics requested in the TFTs differ to some degree. For example, the pixel TFT functions as a switching element, and is driven by applying voltage to liquid crystal. Since liquid crystal is driven by an alternate current, a system called “frame reversing drive” is frequently employed. In the system, the characteristics requested of the pixel TFTs is a sufficiently decreased OFF current value (that is, the drain current flowing when the TFTs are turned off) in order to suppress power consumption to a sufficiently low level. On the other hand, since a high drive voltage is applied to the buffer circuit of the control circuit, it is necessary that the withstand is sufficiently increased so that it is not broken when a high voltage is applied. Further, in order to increase the current drive capacity, it is necessary to sufficiently secure the ON current value (that is, the drain current flowing when the TFT is turned on).
Also, in order to control the threshold voltage (hereinafter called “Vth”), which is an important characteristic parameter in the TFTs, in a specified range, it was necessary to decrease a charge defect population of the underground film, gate insulation layer and interlayer insulation film, which are formed of an insulation layer in close contact with the active layer, in addition to valence electron control of the channel forming region, and it also was necessary to take the balance of the internal stress into consideration. With respect to such a request, a material containing silicon such as a silicon oxide layer and a silicon oxide nitride layer, etc., as a component element was suitable.
Thus, in an attempt to improve the performance of a monolithic type liquid crystal display device, an attempt to improve the performance of TFTs by only increasing the crystalline grain size of a crystalline semiconductor layer forming the active layer is insufficient, and it was necessary to take various characteristics of the active layer, the underground film, gate insulation layer and interlayer insulation film, which are formed above and below the active layer, into consideration.
The present invention is a technology to solve these and other problems. It is therefore an object of the invention to achieve a semiconductor area formed of island-like patterns formed to be mono-crystalline or an area which can be regarded as mono-crystal, and at the same time, to achieve a lamination structure which can stabilize various characteristics of the TFTs. Further, in a semiconductor device represented by a monolithic type liquid crystal display device in which a plurality of circuits are formed on the same substrate, wherein arrangement or orientation of TFTs having adequate properties is enabled on the basis of the specifications requested by the function circuits thereof, and the operation characteristics and reliability can be achieved remarkably well.